The warm temperatures experienced over the winter have resulted in above normal Lake Michigan surface water temperatures this year. The below graph from NOAA Coast Watch shows the average surface water temperature for Lake Michigan so far this year (red line) is running about 1-1.5 degrees C above the 30 year average from 1992 to 2011. This above normal trend is likely to continue through March, most likely returning to near normal temperatures in April and May as the lake begins to warm.
Below is a MODIS image depicting the lake surface temperature over lower Lake Michigan and western Lake Superior just before 11 am on March 10th. Lake surface temperatures were in the middle 30s to around 40 over lower Lake Michigan. You can also see a few smaller lakes highlighted in central and southern Wisconsin. Other lakes located farther north are not visible in this image because they are iced over. However, with temperatures expected to peak at 25 to 30 degrees above normal this week, more inland lakes will become visible in the satellite image. The warmer weather will likely continue over the next several weeks as well, allowing Lake Michigan to begin its annual warm-up.
NWS - Milwaukee/Sullivan